Speedometer not accurate, engine revs

Hello, this is my first post and not entirely certain if I am posting to the correct area.

I have a 1998 Mustang Cobra. I had a new transmission installed and I believe it was for a later model. The speedometer is registering 4x faster than what I am actually moving. The miles are not registering either but that is not that important to me now.

The issue I have is the engine wants to rev about 600 rpms over the current rpms when I push the clutch lever in. For example, I am moving at about 50mph (or any speed above 20mph) and put the transmission in neutral and coast, what happens next is the engine will rev from lets say 1000 rpms (as I am coasting) to 1600 rpms but only for a split second and then go back to 1000 rpms and continue the cycle until I stop. There is about a 1 second delay from the time the rpms drop back down to 1000 and go back up to 1600 for a split second. The car idles fine at about 800rpms.

I am wondering if the revving is somehow associated with the incorrect speedometer reading (4x faster than I am moving). If I get the speedometer fixed then I wonder if the revving issue will go away.

Hey I know exactly what your getting at, my 2000 GT had a 97 t45 in it , my speedo wouldn't register at all. Because, the BorgWarner and tremec in 96-98 were gear driven speedos, the later model 99+ used a reluctor ring on the out put shaft and a magnet reads the rotations. Yes the rpm revving is directly related to the out put of the VSS. As a matter of fact my 2000 had the reverse problem your having. Upon releasing the accelerator, and coasting my rpms would drop and effectively create an engine brake.

There are 3 methods you can take here to remedy the situation .... I chose to put the correct trans in mine.

1. get the correct model year trans for your car.

2. Swap out the tail housing and remove the magnetic reluctor ring and replace with a gear driven , get a gear driven vehicle speed sensor. The tail housing must come from an earlier year T45 and must be swapped due to the way the sensor mounts in each.

If you choose to swap the rear tail housing and speedo gear, I have a 97 borg tail housing and gear drive for it , it might even still have the VSS in it for that year. PM me if your interested in purchasing it. Id make ya a good deal on it.

Hi Dawson and thank you so much for your reply! I finally am getting some ground on this issue I have had for a long time. I have not cared much to fix it but now I do. I just had a complete engine overhaul. See the pretty pics

Yes, I notice this. It is very annoying. It was much worse when the speedometer was connected and the odometer turning. The engine would stall out all the time until I disconnected the speedometer and the engine braking was not as strong either.

Question: Does the 1998 Cobra have a T45 tranny? I didnt think it did.
Question: Do you know if damage is being created within the tranny due to this VSS issue?
Question: What is the tail housing? Its not the differential gears, correct?

96-00 GTs and Cobras used a T45 trans, in 2001 tremec changed over to the 3650.

Your not incurring any damage to the transmission , you may be fouling out your plugs a little if the engine is not calculating fuel and timing as it should. That may have even contributed to your high carbon deposits on the valves. The PCM uses Throttle position, engine rpm , ambient air temps , engine temps , and vehicle speed to determine proper Air/Fuel mix , and correct ignition timing.

The tail housing is the rear part of the transmission. It can easily be removed with the trans still in the vehicle. Its easier with it out but can be done in the vehicle.

The GTs used a 7 tooth drive gear for the speedo , while the cobra used an 8 tooth drive gear for it. Most likely due to a difference in rear gear selection.

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